Electroplating tantalum



Patented Oct. 31, 1933 UNITED- STATES PATENT OFFICE ELECTROPLATING TAN'IALUM Pennsylvania No Drawing. Application July 26, 1930 Serial No. 471,081

Claims.

This invention relates to the preparation of -metals and more particularly to the preparation of rare refractory metals by the electrolytic decomposition of rare refractory metal compounds 5 in fused baths.

In compending application Serial No. 275,264 filed May 4, 1928 and entitled Method of producing tantalum and other rare refractory metals by electrolysis of fused compounds, there is disclosed a method of producing tantalum metal powder by the electrolytic decomposition of a fused bath comprised of alkali metal halides containing a proportion of a double alkali metal tantalum halide compound. The tantalum metal powder thus prepared is recovered from the fusion mixture by lixiviation methods and subjected to a vacuum sintering process to render the same coherent. The coherent tantalum metal thus obtained is substantially pure and may be readily 2o worked down into articles such as wire, rod, sheet,

filament and the like.

Heretofore in the arts tantalum has been applied to many uses. Because of its chemical inertness and high melting point it has been proposed as a substitute for platinum in crucibles and similar chemical ware.

In electric discharge devices, such as electron discharge devices of the evacuated type, gaseous conduction devices, radio tubes, X-ray tubes, in-

candescent lamps and the like it has been proposed to employ tantalum as electrode material,

gas clean up agent, filament and the like. The successful application of tantalum as an electrode or as a gas clean up agent and filament is restricted, however, by reason of the fact that the tantalum lacks the rigidity which is characteristic of the refractory metals, tungsten and molybdenum, for example. The physical characteristics of tantalum which permit its ready mechanical deformation to desired shapes and sizes limit the application of the same in the arts.

It is one of the objects of the present invention to facilitate the use andapplication of tantalum in'the arts. It is another object of this invention to provide tantalum surfaced materials which are useful in the arts.

It is another object of this invention to provide a process for electroplating tantalum upon metal surfaces. I Other objects and advantages will become apparent as the invention is more fully disclosed. In copending application Serial No. 275,264 above identified there is disclosed a process for (Cl. cor-19)" depositing tantalum metal powder upon a cathode. In the practice of the invention set forth in this copending application it was one of the objects to prepare the tantalum in a reasonably coarse state, loosely adherent to the co cathode so that the same may be subsequently removed therefrom by mechanical means, such as scraping. In orderto obtain this type of deposit upon the cathode current densities ranging from 0.5 to 1.0 amperes per cm (50-100 amperes per decimeter were employed.

We have found that when current densities of a much lower order are employed that the tanta-= lum thus deposited may be made to adhere to the cathode surface to form a smooth continuous film thereon which is substantially impervious to acids and to gases.

As a specific example of the present invention we form an electroplated tantalum coating upon iron, nickel, molybdenum, tungsten and the like is surfaces in the following manner:

A fusion mixture comprising potassium chloride, potassium fluoride, potassium tantalum fluoride and tantalum oxide in the following relative proportion is employed: so)

300 grams -l KCl 120 grams KF 100 grams KaTaF'z 25 grams TazOa This fusion mixture is placed in a graphite crucible and brought to fusion by any convenient means. The approximate melting point of this bath is 700 to 750 C. The bath is maintained during electrolysis at a temperature approximat- 9o ing 750 C. The base metal to be surfaced with the tantalum is immersed in the fused bath and allowed to remain therein until it has attained approximately the same temperature as the bath. Due to the corrosive action of the fusion mixture the surface of the immersed metal will be substantially cleansed of all deleterious oxides. It is better, however, to subject the metal prior to immersion to the usual surface cleansing processes, such as subjecting the metal body to the action of a reducing gas'such as hydrogen, etching the surface with acid or alkali solutions or sand blasting. The latter method is to be preferred as the roughened surface materially facilitates the retention of the deposited tantalum thereon.

The metal base thus prepared and immersed in the bath is given a final cleansing by making the same temporarily the anode in the bath so as to subject the same to anodic solution effects and desced to relatively high temperatures in the then is made the cathode. A current density of approximately .01 to 0.1 amperes per cm (1 to 10 amperes per decimeter is employed in effecting electro-plating of tantalum to the base metal.

The specific current density employed will vary materially with the different base metals bein plated as will also the time interval necessary to effect a reasonably good deposit.

As an illustration reference should be made to the following table wherein is given some representative current densities employed in the surfacing of nickel, iron and molybdenum articles with tantalum.

Ampere Base metal cm, in per Time minutes Typg of bath cm per coating 14 217 15 3. 0 Too coarse 10 .05 10 5 Good 12 041 15 62 Best 8 062 30 1. 8 Good 18 028 l. 5 Good The above table illustrates the type of coating obtained in the plating of strips of sheet metal with different current densities. In the plating of shaped articles, such as crucibles, radio tube plate (or anode electrodes) or the surfacing of lengths of wire, filament and the like, variations in the current densities that may be employed and in the arrangement of elements within the bath must be made.

In the surfacing of a crucible, for example, the anode electrode must be shaped in the approximate convolution of the crucible. In the surfacing of a radio tube plate electrode it is preferable to electroplate the surface of a sheet of the base metal, such as nickel or molybdenum and thereafter shape the electrode therefrom. In the surfacing of wire, filament and the like it is preferable to do the same in a continuous manner providing means exterior to the bath for cleansin the filament prior to entering the bath, said means depending primarily upon the nature of the base filament employed.

The type of coating obtained by the practice of the above identified process can best be ascertained by immersing the coated metal in hot dilute nitric acid for an interval of time sufiicient to indicate whether the base element is being attacked by the acid. The tantalum is not touched by this acid.

We have found that when a nickel or iron sheet or crucible shaped article has been so surfaced with tantalum that the article maybe incanopen air without substantial oxidation or deterioration. The articles also may be subjected to measw 1. The method of forming a continuous adhervent electro-plated tantalum surface upon an electrically conductive body comprising immersing said body in a fused bath comprised of alkali halide compounds containing a proportion of an ionized tantalum compound and electrolytically depositing the tantalum thereon at relatively low current densities ranging from 1 to 10 amperes per square decimeter.

2. The method of electrolytically depositing an adherent coating of tantalum upon an 'elctrically conductive body comprising immersing said body in a fused bath comprised of alkali halide compounds containing a proportion of an ionized tantalum compound and electrolytically depositing the tantalum thereon at relatively low current densities ranging from 1 to 10 amperes per square decimeter.

3. The method of electroplating an adherent continuous tantalum film comprising immersing an electrically conducting body in a fused bath comprised of alkali halide compounds, a proportion of a double alkali metal-tantalum fluoride compound, and electrolytically depositing the tantalum on said body at relatively low current densities ranging from 1 to 10 amperes per square decimeter.

4. The method of electroplating an adherent continuous tantalum film comprising immersing an electrically conducting body in a fused bath comprised of potassium chloride and potassium fluoride compounds, a proportion of potassium tantalum fluoride and a proportion of tantalum oxide,'and electrolytically depositing the tantalum on said body at cathode current densities ranging from 1.0 to 10.0 amperes per square decimeter. 5

5. As an article of manufacture, an electri= cally conductive body surfaced at least in part with a relatively smooth, continuous film of electrolytically. deposited tantalum, said film being in the form of an even coherent layer which is firmly adherent to the conductive body.

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